Classification of Lichens | Lichenology

In this article we will discuss about classification of lichens based on Morphology, habitat, internal structure and fungal partner.

Classification of Lichens

• A lichen is not a single organism, it is a symbiotic association between a fungus and algae.
• There is no natural classification system for lichens.
• They are classified based on morphology, habitat, internal structure and fungal partner.

1. Classification of Lichens (Based on Morphology)

• On the basis of their morphology, lichens are classified into three main groups: crustose, foliose and fruticose lichens

I. Crustose Lichens (Encrusting lichens)

• These lichens found as a thin or thick crust over rocks, soil or tree barks.
• They are very difficult to separate from their substrate.
• The thalli may be fully or partially embedded, So that only the fruiting bodies visible above the surface.
• E.g., Lecanora, Graphis, Rhizocarpon, Ochrolechia etc.

II. Foliose Lichens (Leafy lichens)

• These lichens have a flat, expanded, leaf like thallus (commonly grayish or brownish in colour), that spreads out horizontally over the surface.
• They attach to the substrate using rhizoid-like outgrowths called rhizines.
• They can be easily removed without damaging the substrate.
• E.g., Physcia, Parmelia, Gyrophora etc.

III. Fruticose Lichens (Shrubby Lichens)

• These are the upright or hanging lichens.
• They have a branched, bushy thallus that can hang from the substrate.
• The thalli may be erect or pendant.
• These are flat, cylindrical or ribbon like, well branched and resembling small shrubs.
• These lichens are attached only at the base by a flat disc and can be easily removed by hand.
• E.g., Cladonia rangiferina, Usnea barbata etc.

2. Classification of Lichens (Based on Habitat)

• On the basis of habitat lichens can be divided into following categories.

1. Muscicolour (Mosses): These lichens grow along with mosses. E.g., Cladonia.
2. Foliicolous (Leaves): These lichens are found on the surface of leaves. E.g., Calicium.
3. Terricolous (Soil surface or Ground): These lichens occur on the soil surface, particularly in hot climates with sufficient rain and dry summer. E.g., Cladonia, Florekeana, Lecidea, Collema etc.
4. Saxicolous (Rocks or Stones): These lichens grow on rocks and stones surface. E.g., Dermatocarpon, Xanthoria, Verrucaria etc.
5. Corticolous (Tree barks): These lichens grow on the barks of trees, primarily in subtropical and tropical regions. E.g., Parmelia, Usnea, Grpahis.
6. Lignicolous (Wood): They are grow directly on the wood. E.g., Calicium etc.
7. Marine Lichens: They are grow on siliceous rocky shores of sea. E.g., Verrucaria, Caloplaca etc.
8. Fresh water lichens: These lichens grow on siliceous rocks in fresh water. E.g., Epheba, Hymenelia etc.

3. Classification of Lichens (Based on Internal Structure)

• On the basis of internal structure the lichens are divided into two groups: Homoiomerous lichens and Heteromerous lichens

I. Homoiomerous lichens

• In these lichens, the fungi and algae are not differentiated into distinct layers of tissue. Instead, algal cells are randomly distributed throughout the thallus.
• E.g., Collema and Leptogium

II. Heteromerous lichens

• These lichens are differentiated into several layers of tissues.
• A cross section of the heteromerous lichen shows four distinct zones: Upper cortex, Gonidial layer (Algal layer), Medulla and Lower cortex.

1. Upper Cortex: This layer forms the upper surface of the thallus. It is thick, protective and composed of tightly interwoven fungal hyphae, typically lacking intercellular spaces.
2. Gonidial layer: This layer consist of algal cells and they are surrounded by thin walled, loosely packed fungal hyphae.
3. Medulla: This layer constitutes the bulk of the thallus and made up of loosely packed, thick-walled fungal hyphae.
4. Lower Cortex: This layer consists of closely packed, dark coloured hyphae. From the lower cortex, rhizinae arise, which serve to attach and absorb nutrients.

• E.g., Physcia and Parmelia

4. Classification of Lichens (Based on Fungal Partner)

• The fungal partner primarily belongs to ascomycetes, with some basidiomycetes and rarely deuteromycetes.
• On the basis of the fungal partner, lichens are classified into three groups: Ascolichens, Basidiolichens, and Deuterolichens.

I. Ascolichens

• In this lichens, the fungal component is belongs to Ascomycetes.
• The sexual reproduction of Ascolichens is similar to Ascomycotina.
• They produce ascus with ascospores during the sexual reproduction.
• More than 95% of the lichens fall into the Ascolichens
• These lichens are further divided into two subgroups:
• Gynocarpeae: Here, the fruiting body (ascocarp) is an apothecium. E.g., Parmelia.
• Pyrenocarpeae: In this subgroup, the fungal fruiting body (ascocarp) is an perithecium type. E.g., Dermatocarpon.

II. Basidiolichens

• In basidiolichens, the fungal component belongs to basidiomycetes.
• The sexual reproduction of basidiolichens is similar to basidiomycotina
• They produce basidia with basidiospores during sexual reproduction.
• Only a few lichens (four genera reported so far) are classified as Basidiolichens.
• E.g., Dictonema, Corella and Cora

III. Deuterolichens

• They are also known as lichen imperfectii.
• They have fungal partners from the Deuteromycotina division.
• These lichens do not undergo sexual reproduction or should say that lichens with sterile thalli are constituted by this group.
• E.g., Lepraria, Leprocaulo, Crysothrix.